This invention relates to a needle-free or needleless hypodermic injection device used to subcutaneously inject medication into the skin of a patient.
There are many attributes which are highly desirable in an injection system, whether it is of the type that uses a needle and syringe, or is of needleless configuration. It is necessary to accurately meter a dose, and to permit the dose to be varied from injection to injection. It is also highly desirable that the injection system be capable of use by the patients themselves, and be usable by someone with limited physical capabilities. For example, physically disabled patients are often in need of regular medication. Some patients have particular difficulty with motor control, and yet to live an independent life, they need to be able to self-administer injections. Needleless injection systems are suitable for these types of patients because one of the common problems is that it is difficult for them to grasp and manipulate a smaller object such as a syringe. It is also desirable that the system can be used, that is, loaded, manipulated, and administered, by one who has had no medical training and who has had little training or experience in the use of the injection system.
In a needleless injection system, there are a number of attributes which need to be considered which are not necessarily present when a conventional syringe and needle-type system is used. For example, the needleless injection system needs to be positioned directly against and roughly perpendicular to the skin of the patient. This is not a factor when the skin is being pierced with a needle. Also, any needleless injection system using compressed gas needs to have adequate compressed gas loading pressure in order to prevent misfiring or improper application of medication. Moreover, the loading of pressure within the injection system needs to be performed in a predictable, repeatable fashion so that the pressure is precisely loaded. Some existing equipment includes a turn down screw which pierces a compressed gas vessel. But during the turning down operation, gas pressure is lost, and the amount of lost pressure varies directly with the rate at which the screw is turned. This will normally not affect the proper operation of the injector, but it may well reduce the number of injections possible with a single cartridge.
Another desirable feature in needleless injection systems is that the units be relatively inexpensive, be virtually maintenance free, and be able to last for an appropriate period of time. Not only are patients who utilize these products often supported by fixed incomes, but because they are often disabled, they may not recognize that there is a problem with a unit which is prone to breakdown or which needs regular maintenance. Therefore, reliability is an important advantage, as well as the ease of use previously discussed.
One problem which exists with some of the prior injection systems is that it is difficult to fill the ampule with a precise measurement of medication in a process which is simple and which, again, may be performed by someone who has difficulty handling small objects or who is otherwise disabled.
One of the drawbacks with some conventional gas powered injection devices is that the gas pressure acting upon the unit may rise too slowly, so that the medication initially ejected from the injector does not have sufficient pressure or velocity to pass through the skin. In that instance, it is possible that a portion of the medication will splash back onto the patient, so that the patient does not receive a full dose. In these instances, it is virtually impossible to determine how much of a particular dose has been injected, so the user, who is typically unskilled and not very knowledgeable in these systems, may not be able to determine whether another injection is necessary. This therefore results in the patient either being undertreated, or perhaps even causing an overdosage in the event the patient unnecessarily repeats an injection.
Another drawback with prior injection systems, including those which are needleless, is that a number of conditions have to be satisfied for the system to work properly. It would be helpful if the user is given some warning if one or more of the conditions is not satisfied. While an interlock could prevent the system from firing in those circumstances, it would be even more helpful if the user was given some sort of warning prior to attempting to initiate the injections. Such warnings are typically not provided in prior art injection systems.